Question: On the first 3 quizzes of his geometry class, Christopher got an average score of 81. What does he need on the next quiz to have an overall average of 83?
Answer: Let his score on the next quiz be $x$ The sum of all of his scores is then $3 \cdot 81 + x$ The same sum must also be equal to $4 \cdot 83$ Solve: $x = 4 \cdot 83 - 3 \cdot 81 = 89$.